1. Field of the Invention
This invention relates to a method of producing structural formula PrBa.sub.2 Cu.sub.3 O.sub.Y single crystal exhibiting superconductivity and a superconducting device constituted solely of PrBa.sub.2 Cu.sub.3 O.sub.Y crystal materials at least one of which is PrBa.sub.2 Cu.sub.3 O.sub.Y crystal exhibiting superconductivity.
2. Description of the Prior Art
It is generally held that substances of the so-called 123 structure represented by the structural formula RBa.sub.2 Cu.sub.3 O.sub.Y (R=rare earth element, Y=7-y) exhibit superconductivity in the temperature range of 80-90 K, giving hope of their usability to configure superconducting devices that can operate even at the temperature of inexpensive liquid nitrogen. The conventional view has been, however, that when the rare earth element is Pr, i.e., in the case of PrBa.sub.2 Cu.sub.3 O.sub.Y, superconductivity is not exhibited despite the 123 structure. The PrBa.sub.2 Cu.sub.3 O.sub.Y single crystal produced by the floating zone method reported earlier by the inventors in Japanese Patent Application No. 6-531725 (Example 3), for example, did not exhibit superconductivity.
Up to now, therefore, proposed uses of PrBa.sub.2 Cu.sub.3 O.sub.Y have been limited to those that utilize its non-superconducting properties (insulating, semiconductor and normal conducting properties) in such applications as a tunnel junction barrier or interlayer isolation material in Josephson devices (superconducting devices using the Josephson effect) utilizing thin film of another type of 123 structure. It has been considered impossible to configure Josephson devices and the like from PrBa.sub.2 Cu.sub.3 O.sub.Y either in its pure form or as doped with a small amount of some other element. (No such device has yet been reported.) It is known, however, that when an oxide high-temperature superconducting Josephson device is fabricated as a laminated structure consisting of different types of thin film or using a combination of substrate materials and includes non-superconducting PrBa.sub.2 Cu.sub.3 O.sub.Y thin film as only a part thereof, difference in lattice constant and/or coefficient of thermal expansion produces distortion at the junction interface, a critical part of such a device, and leads to decreased performance owing to degradation of electrical resistance and superconducting properties.
Further, in a superconducting three-terminal device utilizing superconductor-insulator transition produced by controlling the carrier (electron and hole) density in the oxide high-temperature superconductor from the outside, the amount of change in superconductivity for a given change in carrier density is small, making it difficult to fabricate a useful device.
In order to overcome these difficulties, the inventors demonstrate that, contrary to common belief, PrBa.sub.2 Cu.sub.3 O.sub.Y exhibiting superconductivity exists, propose a superconducting device consisting solely of PrBa.sub.2 Cu.sub.3 O.sub.Y, namely of non-superconducting PrBa.sub.2 Cu.sub.3 O.sub.Y and superconducting PrBa.sub.2 Cu.sub.3 O.sub.Y in combination, and further propose a specific method of producing superconducting PrBa.sub.2 Cu.sub.3 O.sub.Y.
Through extensive research, the inventors discovered that conventionally produced PrBa.sub.2 Cu.sub.3 O.sub.Y single crystal does not exhibit superconductivity because the crystal production method based on conventional technology deprive it of this property.
Specifically, since production of superconducting PrBa.sub.2 Cu.sub.3 O.sub.Y single crystal requires further improvement of single crystal quality, it is highly important to eliminate causes obstructing manifestation of superconductivity from PrBa.sub.2 Cu.sub.3 O.sub.Y single crystal produced by the prior-art floating zone method to the utmost possible. For this, it is necessary to ascertain crystal production conditions for PrBa.sub.2 Cu.sub.3 O.sub.Y that enable crystal growth to progress stably. Important factors include the atmosphere conditions and the crystal growth rate. PrBa.sub.2 Cu.sub.3 O.sub.Y crystal production conditions for the purpose of obtaining a superconductor have not been reported to date.
In light of the foregoing circumstances, the inventors conducted repeated single crystal production experiments with regard to PrBa.sub.2 Cu.sub.3 O.sub.Y. As a result, they discovered crystal production conditions for obtaining PrBa.sub.2 Cu.sub.3 O.sub.Y capable of functioning as a superconductor. Based on the results of this research, this invention provides a floating zone method in accordance with conditions suitable for production of superconducting PrBa.sub.2 Cu.sub.3 O.sub.Y single crystal.